Active Transport/Transcript
Transcript Text reads: The Mysteries of Life with Tim & Moby Tim and Moby are riding down an escalator to the subway from the street level. The scene switches to show turnstiles at the entrance. Tim walks through the turnstile as a green light flashes. Moby tries to follow Tim through, but a red light flashes, along with a buzzing sound. Moby stops at the entrance and frowns. Tim is wearing a shirt with a picture of a kidney-shaped orange figure with four purple spheres inside and one purple sphere outside of it. The purple sphere outside of the orange figure is on a red arrow pointing in toward the orange figure. MOBY: Beep. TIM: Um, I think you might need to use the special entrance. MOBY: Beep. The scene shows a subway attendant named Herman behind a ticket window. Another buzzing sound is heard, followed by a click. A door with bars next to the turnstile opens, and Moby walks through. Moby is still frowning. MOBY: Beep. Moby hands an envelope to Tim. Tim reads the letter inside. TIM: Dear Tim & Moby, what is active transport? From Queen. Funny you should ask. Active transport is one way that substances get in and out of a cell. A diagram of a cell is shown. The cell is a large round object surrounded by a cell membrane, which is represented as a thick layer of alternating diagonal stripes. A number of organelles, or cell features, are visible inside the cell. TIM: In order to do that, they need to penetrate the cell membrane. The view zooms in on a small section of the cell membrane. TIM: Some substances, like oxygen and water molecules, move through the gaps in the membrane without any effort on the part of the cell. Water and oxygen molecules are shown moving through the membrane. A water molecule is represented by a large, round, red sphere (oxygen) that is connected to two smaller white spheres (hydrogen). An oxygen molecule is represented by a pair of the larger round, red spheres. TIM: Because the cell expends no energy, this is called passive transport. Passive transport works by diffusion—that's the natural spreading of particles through a liquid or gas. The term passive transport pops up on the animation. The surface of a blue liquid is shown gently swaying. A single dark-purple droplet falls from above onto the surface, forming a small dark-purple circle. The purple circle widens while growing fainter until the surface is almost back to the original blue color. The word diffusion labels this animation. TIM: Particles move from areas of high concentration to areas of lower concentration. The screen is divided in half by a vertical layer of alternating stripes. The left side of the screen contains a single gray ball and is labeled "low concentration". The right side contains seven gray balls and is labeled "high concentration". While Tim is speaking, three of the balls move from the high concentration side to the low concentration side so that both sides have four balls each. MOBY: Beep. TIM: Yep, this is called moving down a concentration gradient. The scene shows Moby briefly then returns to the divided screen in the initial state, with one ball on the left and seven on the right. The animation of the three balls diffusing repeats, but this time there is a large arrow pointing from the high concentration side to the low concentration side. The arrow is labeled "concentration gradient". TIM: Not all substances can cross a cell membrane without some help. Active transport is when a cell uses its own energy to move molecules across the membrane. MOBY: Beep. TIM: Well, take plants for example. A plant is shown from the side with visible roots extending into the soil. TIM: Root cells in plants often need to absorb minerals from the surrounding soil, even when their cells already contain higher concentrations of those minerals. An inset shows a magnified view of a section of root with visible cells. The cells are generally rectangular and packed like bricks, but the center-most cell in the inset contains a projection extending into the soil. Except for the projection, the inset is divided in half between root and soil. Minerals are represented by small silver balls. There are many more silver balls in the roots than in the soil. MOBY: Beep? TIM: Well, diffusion would cause the cells to actually lose minerals, so the cells have to work to keep their minerals in and take in more. The screen shows a larger sketch of the root cells and soil diagram. The root side is labeled "high concentration". The soil side is labeled "low concentration". A large arrow labeled "concentration gradient" points from the root side to the soil side. As Tim speaks, minerals move out of the roots and into the soil. Then the moving minerals change direction and move back into the roots, along with all of the minerals that had started out in the soil. TIM: To do this, cells use transport proteins, complex molecules lodged in the membrane. The scene shows a magnified view of a small section of a cell membrane. There are many silver balls inside the cell and a single silver ball outside. A transport protein is represented by a purple, notched rectangle with a round central component, spanning the width of the membrane. The notch faces outside the cell. TIM: The transport protein binds with the needed substance, then changes its shape to move the substance inside the membrane, where it's released. The silver ball outside the cell enters the transport protein through the notch and lodges in the center. Then the notch closes up and changes shape so its opening faces into the cell. The silver ball exits the transport protein through the notch and into the cell interior. TIM: In order to work, transport proteins need ATP, the main source of cell energy. A new silver ball enters the scene from outside, and the process of transport through the transport protein begins again. While the substance is bound to the protein, three yellow circles with lightning bolt icons appear in the cell. The yellow circles are labeled "UpperWord A T P". The transport process continues and the substance is released into the cell. MOBY: Beep. TIM: Sometimes cells absorb substances by another form of active transport called endocytosis. The scene changes, showing a new section of cell and cell membrane. The interior of the cell is empty. There is a purple, formless object outside of the cell. TIM: That's when the cell membrane surrounds a substance and engulfs it. The membrane pinches off into a little sphere called a vesicle or a vacuole, and voila—the substance is now inside the cell! An animation shows the process of endocytosis, with the purple object being transported into the cell. TIM: The white blood cells in your immune system absorb and kill germs using endocytosis. The scene switches to the bloodstream. A formless white blood cell moves around. A small, blue, rod-shaped object approaches the white blood cell. The white blood cell surrounds the small rod-shaped object and destroys it. TIM: Cells can expel large molecules through exocytosis. MOBY: Beep! TIM: Right, that's basically a reversal of endocytosis. Substances that need to be released are enclosed in a vesicle, which fuses with the cell membrane. Then whatever is inside the vesicle gets released outside the cell. An animation shows exocytosis as Tim describes it. Tim and Moby are standing on the subway platform. A train enters the station and stops. Tim and Moby attempt to enter the train through the sliding doors at the same time and get stuck in the doorway. Moby frowns. MOBY: Beep. Tim grunts and tries to pull away from Moby, but fails. TIM: Well, here is another fine mess you've gotten us into. Category:BrainPOP Transcripts